Essential tremor is the most common form of pathologic tremor, but little is known about its underlying pathophysiology. Many investigators have speculated that essential tremor emerges from pathological change in the mechanisms of physiologic tremor, but until recently, there was little support for this hypothesis. Mild essential tremor is qualitatively identical to the 8- to 12-Hz component of physiologic tremor. More advanced essential tremor exhibits a frequency of 4 to 7 Hz. Consequently, a negative logarithmic relationship exists between the amplitude and frequency of essential tremor. Based upon these observations, the progression of essential tremor probably entails a gradual reduction in its frequency. A pilot study of 18 patients supported this hypothesis, which will now be tested in a larger cohort of patients. Wrist tremor and forearm electromyograms (EMG), writing tremor and head tremor will be recorded from 100 patients in years 1, 3 and 5 of this grant. The majority of these patients will come from one or more generations of 51 families with essential tremor. Tremor amplitude and frequency will be computed by spectral analysis, and the changes in tremor amplitude and frequency, over time, will be determined. Wrist tremor and forearm EMG will also be recorded from 100 20- to 40-year-old male and female controls. Two groups of controls will be identified: those with and those without a prominent 8- to 12-Hz tremor. The prevalence of a parent with essential tremor in the two groups will be compared. In addition, the prevalence of a prominent 8- to 12-Hz tremor in this cohort of 100 controls will be compared with that in a comparable number of the patients' siblings. These studies will provide important insight into the clinical significance of a prominent 8- to of this tremor to essential tremor. Finally, the essential tremor oscillator will continue to be of essential tremor to sinusoidal and pulsatile a computer-controlled torque motor and provide further insight into the role of the in governing the clinical expression of the, as yet unidentified, neural oscillator of essential tremor.